C4 brake issues

My c4 brake is not holding past 3500 rpm.The converter is a 9 inch 4800 stall. It has no issues going in reverse and does not slip in reverse or any forward gear and has no flare up. It is acting like i push and let go..push let go..if I keep the rpm to 3000 area it works fine.I always foot brake the car and run mostly bracket races with it and just discovered this during tnt I wanted to see what the ole car would do leaving at 4500+. The converter will flash off foot brake at 4500-5000 .
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Check low reverse band adjustment . Band strut should be tightened to 120 inch lbs or 10 ft lbs , then threaded strut should be backed off 2 turns hold and lock the 3/4 lock or jam nut to 40 ft lbs .

If this doesn't help need to check trans brake wiring , solenoid , and then valve body. .

Last a Pressure test should be run. .

This may sound stupid but I had problems with my brake, I don't run a alternator and thought it was a voltage problem. I found that it was due to low fluid level. I read that because the c-4 holds a small amount of fluid when the brake is applied it is doing two functions and uses more fluid to do so and if the fluid level isn't optimal it will not function properly.